Device containing electron tubes for automatically stabilizing high-voltage sources with symmetrical potential distribution



06L 1951 o. NETTELAND DEVICE CONTAINING ELECTRON TUBES FOR AUTOMATICALLYSTABILIZING HIGH-VOLTAGE SOURCES WITH SYMMETRICAL POTENTIAL DISTRIBUTIONFiled Dec. so, 1947 IN VEN TOR.

Fatented Oct. 2, I951 DEVICE CONTAINING ELECTRON TUBES FOR AUTOMATICALLYSTABILIZING HIGH-VOLTAGE SOURCES WITH SYM- METRICAL POTENTIALDISTRIBUTION Olav Netteland, Oslo, Norway Application December 30, 1947,Serial No. 794,572

In Norway November 1, 1945 3 Claims.

The present invention relates to devices for automatically stabilizinghigh voltage sources by means of electron tubes inserted directly in thehigh voltage circuit in series with the load and acting to maintainconstant the output voltage at a predetermined value and simultaneouslyto maintain symmetrical the voltage distribution relatively to anearthed center point or a center point with a fixed potential relativelyto ground independently of the variations of the input voltage.

The devices are especially intended for stabi- I lizing high voltagesources used for the production of radiation, such as X-rays generatorsfor medical or technical purposes, but the same means may also be usedfor other purposes in science and technics, without making any principalalterations therein.

The use of electron tubes in voltage stabilization is not new, a varietyof different circuits being described in the literature some of whichare widely used in practical technics. All different methods, however,are based upon the same fundamental property of grid controlled electrontubes, i. e. that their anode voltage is a function, especially a linearfunction of their control grid potential, thus the tube can be used as asubstitute for an automatically variable ohmic resistance.

This tube operating as a resistance can therefore be inserted in seriesor in parallel with the output load, the grid potential being varied ina suitable way to maintain constant the output voltage.

Electron tube stabilizers are chiefly used in connection with low ormedium voltages, but have also been applied for high voltage in theorder of 100 kv. for producing radiations, especially intended forscientific purposes. Those stabilizers are usually provided with astabilizer tube in the positive branch of the high voltage circuit andare preferably used with the cathode of the electron tube grounded togive a high voltage source with grounded positive pole and negative poleat high tension, an arrangement which is often very advantageous.

In connection with X-ray generators for medical purposes, electron tubestabilizers of the said v type are not so much used, in spite of theexisting need for the highest possible constancy and I exactreproduction of the working conditions.

The decisive reason for this fact is that modern X-ray generatorsusually give a symmetrical high voltage relatively to a grounded centerpoint, an arrangement which is of fundamental importance R3 connectedacross the input of the sta in connection with the so called shockproofX-ray tubes in combination with high voltage cables with earthed metalsheaths. The insertion, of an electron tube as a stabilizing means inthe positive high voltage lead will always give a nonsymmetricalpotential distribution which gives unfavourable working conditions forthe generator as well as for the X-ray tube and its cables.

This nonsymmetrical potential distribution is especially important whenthe high voltage source is a charged condenser which is graduallydischarged by the X-ray tube. In this case the potential drop across thecondenser may be in the order of 30-50 kv. to give a constant outputvoltage during the whole exposure, the stabilizer tube will have tocompensate this total voltage difference and must consequently introducea nonsymmetry of the same magnitude in the output voltage.

If high voltage generators with electron tube stabilization are to becommonly used in medical X-ray technics, it seems necessary to bringforward stabilizer circuits and devices which can give a constant outputvoltage with a symmetrical potential distribution relatively to a,center at fixed potential relatively to ground. The object of thepresent invention is to provide such a stabilizing system which is to bedescribed below with reference to the drawing in which Fig. 1 shows theprincipal diagram of a stabilizer having a stabilizing device in thepositive as well as in the negative lead,

Fig. 2 shows the positive stabilizer, and

Fig. 3 shows the negative stabilizer.

An independent stabilization of the positive and the negative voltage,so that the potential distribution relative to a fixed potential is keptsymmetrical, may be obtained according to this invention by means of twoindependently working stabilizer tubes T1 and T2 as diagrammaticallyshown in Fig. 1. Said tubes are inserted directly in the high voltagecircuit in such a way that T1 with its cathode is connected to thenegative high voltage pole of the generator and with its anode to thecathode of the X-ray tube, whereas the anode of T2 is connected to thepositive pole of the generator with its cathode connected to the anodeof the X-ray tube.

The stabilizer tube T1 is controlled by means of an electronic amplifierF1 of linear character istic', the input of which is connected tovariable contact of a non-inductive voltage divid system. The secondstabilizer tube T2 is com trolled by means of another electronicamplifier 3 F2 the input of which is connected to the variable contactof a non-inductive voltage divider Rt, Rs connected across the output ofthe stabilizing sys tem.

This arrangement acts in such a way that the potential of the respectivecontrol grids is varied in a proper way by means of the independent D.C. amplifiers F1 and F2 with the consequence that the potential dropacros the stabilizer tubes is varied automatically in such a way thatthe output Voltage (the voltagedrop' across the X- ray tube) ismaintained constant, and simultaneously a symmetrical potentialdistribution relative to a grounded center, or a center with a fixedpotential to ground is obtained. According to this principal arrangementboth stabilizer tubes including their necessary amplifiers and otherequipments are as a Whole kept at a high negative or positive potentialrespectively rela-' tively to the symmetry'center of the system and areconsequently surrounded by electrostatically shielding metal boxes whichare maintained at a high potential and preferably connected with thecathodes of the stabilizer tubes T1 and T2 respectively.

Let the unstabilized but symmetrical generator voltage be U11 and thestabilized output voltage be E as in Figure l, the condition forsymmetrical potential distribution in the output voltage is that thepotential drop across the tubes T1 and T2 in every moment is equal toWhen Uh varies the potential difference will have to be distributed inequal amounts on the two tubes, but this demand is not absolute. For allpractical purposes a symmetrydegree of 35% will sufiice but the totalvariations in the output Voltage E must be much's'maller than this,frequently in the order of 0.1% or even less than 0.01% for specialscientific purposes.

According to the above described principal manner of operationthecontrol gridsof the tubes are to be supplied with a varying voltagecausing the anode voltage to vary in a predetermined manner to fulfillthe demands as to stability and symmetry. For both stabilizer units thiscontrol is obtained by means of high 'volttge potentiometers R2, R3 andR4, Rs connected across the unstabilized input and the stabilized outputrespectively. The variable voltages from these potentiometers are partlycompensated by means of constant potential sources and amplified by thetwo independently Working D. C. amplifiers F1 and F2, the resultingvoltages preferably being applied to the tubes T1 and T2 as negativegrid potentials.

According to the invention the stabilizer units are acting independentlyof each other and in a principally different manner. The negativestabilizer unit will have to be a balanced system dependent on thepotentiometer ratio R2, R3, the gain f1 of the amplifier and theamplification factor 1 of the tube T1. The positive stabilizer unit iscontrolled by negative feed back and is accordingly auto stable and doesnot need any critical tuning. The setting of the potentiometer ratio R4,R5 has only a small influence upon the degree of stability, but is aconvenient method in regulating the stabilized output voltage.

In this paper the two stabilizer units have been named the negative andthe positive stabilizer respectively and will be separately described indetail with reference to theFigures 2 and 3. i

a. The negative stabilizer In Fig. 2 a complete circuit is shown for anegative stabilizing system according to the general principles asdescribed above. T1 is the electronic stabilizer tube the cathode ofwhich is connected in series with an ohmic resistance R1 placed in thenegative high voltage lead. Ta is an electronic amplifier tube thecathode of which is connected with the cathode of stabilizer tube T1. Reis a load resistance connected between cathodes of tubes T1 and T3, andthe anode voltage source B2, in the'embodiment shown a voltage regulatedpower supply in which the voltage drop across the resistance Rs may bepartly compensated by the voltage drop across one or more glow dischargelamps in the power supply unit. R2, R3 is the control voltage dividerand B1 a battery or other constant voltage source shunted by apotentiometer P1 to give a suitable grid bias of the amplifier tube T3.By varying the potentiometer P the grid bias of the stabilizer tube Timay also be adjusted to a decided value.

The cathode of the stabilizer tube T1 is principally connected with thenegative high voltage pole of the generator'and i suitably controlled insuch a manner that in a fixed ratio, thevariations of the unstabilizedinput voltage Us are applied to the controlgrid of the tube in phaseopposition. As mentioned above'this is attained by means of a preferablynon inductive potentiometer R2, R3; the potential drop across theresistance R2 is partly compensated by the battery B1 or 'a similarconstant potential source and is applied to the D. C. amplifier F1.' Theamplified and phase inversed output voltage of this amplifier issuitably compensated and applied to the control grid of the tube T1 asa' preferably negative grid potential, which will have to vary in'thedecided way. i

In order to obtain the stabilizing efiect aimed, it is necessary thatthe amplification factor of the tube T1 can be considered as constantwithin the range of regulation, and assumed the anode current ismaintained coristantfthe potential drop U1 across the tubewillbe anapproximately linear function of its grid potential Ugl. If F1 has alinear working characteristic and a constant gain f1, the grid voltage Uof the stabilizer tube consequently is a linear function of thepotential drop across theresistance R2, which is equal to:

R2 rt s and by critically adjusting R2 according to the equation:

(where the sum resistance R2+Ra is approximately constant), the negativestabilizer will compensate 1/11 of the input voltage'variatlons. If theinput voltages is exactly symmetrical, n is equal to 2, and mostly itwill diflef'only'slightly from this value. But it is principallypossible to obtain a symmetrical output voltage even if theinput'voltage is considerably non-symmetrical Above it has beenpresupposed that the load current I be constant but this is the caseseldom only. To a certain degree the effect of small currentvariations'rnay be reduced by usinga stabilizer tube with a'highmutual'co'nducta'nce S1 and a low amplification factor. However,generally a special automatic compensation is necessary to obtain avoltage drop across the tube,

which is sufiiciently independent of the load current. According to thepresent invention, for

. this purpose, a variable ohmic resistance R1 has been inserted betweenthe negative terminal of the resistance R2 and the cathode of the tubeTi, and across this resistance the load current I will produce apotential drop I.R1 acting to alter the effective grid potential of theamplifier tube Ts.

By adjusting the resistance R1 according to the equation:

m-sl (2) and when working on a part of the operating characteristic ofT1, within which its mutual conductance S1 may be considered asconstant, it may be obtained that its grid voltage will vary linearly asa function of the tube current I, in such a manner that the anodevoltage (U1) of the tube will be approximately independent of the anodecurrent. In the special case, shown in Figure 2, the total potentialdrop I.R1 is applied to the grid of T3, and m is equal to one.

Mostly it is difiicult to obtain exact balancing of the system accordingto the Equations 1 and 2, or the balance will be exact within rathernarrow limits only. However, as mentioned above, the voltage symmetry inpractice mostly is not very critical, differences of a few percent beingallowed. Such a degree of stability easily can be obtained, and ingeneral the adjustments of the resistors R1 and R2 can be done once forever. When the variations in load current I are very great, S1 will notbe sufficiently constant, and it is preferred to vary R1 in a few coarsesteps in connection with the regulation of the X-ray tube current tocover the total operating range.

The potential drop (U1) across the tube T1 may be varied at will toobtain proper working conditions by regulating the compensating voltage(B1) defining the grid potential of the amplifier tube T3 andconsequently the anode voltage of the stabilizer tube T1.

By this means it is possible to obtain a symmetrical output voltage atdifferent values of E without disturbing the fixed adjustment of thepotentiometer ratio R2, R3.

b. The positive" stabilizer The positive stabilizer will now bedescribed with reference to Fig. 3.

As mentioned above, the positive" stabilizer is controlled in theordinary way by negative feed back by means of the potentiometer R4, R5

which is connected across the terminals of the stabilized output voltae.- The principal operation of this type of electron tube stabilizer iswell known and need not be described here.

Difierent types of amplifiers may be used even the symmetricalstabilizer, but on account of .totalgain of the amplifier F2 be Ex, ,u:and f2 6 respectively, the stabilizating efiiciency of the system isapproximately:

To obtain a good stabilization it is therefore preferable to use arelatively high compensation voltage as well as a high gain amplifierand a tube T2 with a high amplification factor; the special amplifierneeded should have the following specifications: D. C. amplifier withoutphase inversion, high amplification, high input compensation potential,elimination of special input and output compensation voltage sources,and if possible all the necessary operating potentials delivered from asingle, stable voltage source which is independent of load variations.

An efficient amplifier which fulfil the above mentioned requirements, isshown in Figure 3 in its principal form.

The several stages of the amplifier are all driven by a single,stablized voltage source with a constant potential Ek, its positive polebeing con nected to the cathode of the stabilizer tube T2. The cathodeof the input tube T4 is connected to the negative terminal of the anodevoltage source and its control grid is connected directly to the controlpotentiometer R4, R5, across the resistance R5 of which is produced apotential drop of nearly the value Ek which gives a suitbale input tubegrid potential. In this manner a special input compensation voltagesource is avoided, and El: may easily be given a value which issufficiently high to produce an eflicient stabilization.

The output stage of the amplifier is a D. C. differential stage with twopreferably identical tubes connected in parallel and with a commoncathode resistance R8, the negative terminal of which is connected tothe negative pole of the anode voltage source. In the following thetubes will be named output tube T5 and control tube T6 respectively. InFigure 3 the control grid of the output tube is connected to the anoderesistance R1 of the input tube T4 with its grid controlled in anordinary manner. The grid of the control tube has a fixed potentialrelative to the negative pole of the voltage source and is connected tothe voltage divider R11, R12. To obtain maximum sensibility the anodevoltage of the control tube preferabl should be maintained constant bydirectly connecting the anode to the positive pole of the voltagesource.

The value of the anode resistance R9 belonging or the output tube ischosen so that the mean anode current delivers directly the negativegrid bias for the stabilizer tube T2.

The operation of the amplifier described above is as follows: A voltagevariatio applied to the input tube T4, and the voltage variation fromthe first stage, now amplified and phase inversed, is applied to thecontrol grid of the output tube to cause an anode current variation inthis tube and an inversed current variaion of about the same amount inthe control tube T6, whereby the common cathode is maintained at anapproximately constant potential. The same resulting effect can also beobtained by the use of a single tube in the output stage, connecting itin series with one or more neon tubes inserted between its cathode andthe negative terminal of the voltage source, providing the output tubewith a suitable constant cathode potential.

c. Output voltage regulation As pointed out above, the regulation of the'7 output voltage of the total stabilizer device may be performed byvarying the potentiometer ratio R4, R in the negative feed back controlsystem of the positive stabilizer, maintaining the compensation voltageE1; constant.

According to this method the system will attain a new stable conditionat a new output potential E, the total voltage variation being absorbedby the positive stabilizer tube T2, which will be given a new mean anodevoltage. In this variation, the negative stabilizer tube T1 will nottake part, as the two units are working independently. To obtain asymmetrical output voltage it is necessary to have a simultaneousregulation of the potential drop across the negative tube T1. Asmentioned above, this may be done by varying the input compensationvoltage of the amplifier F1, without disturbing the balance of thesystem.

The principle of simultaneous voltage regulation is illustrated inFigure 7a, in which each position of the sliding contact K1 of thepositive voltage regulator corresponds to a posi.ion of the slidingcontact K2 on the negative potentiomete P1; and when moving the twocontacts synchronously, a symmetrical ouput potential may always beattained. The two regulators may have a resistance characteristicallowing continuous voltage regulation, but in practice it is generallymore convenient to have a regulation in steps, corresponding steps beingadjusted to fulfil the symmetry conditions.

In Figure 7b is shown an arrangement allowing continuous regulation bymeans of a step switch in combination with a fine regulation between thesteps.

As shown in the figure each coarse regulator may have two parallelymounted identical contact fingers K1 with the resistances insertedbetween corresponding contact points, the regulators havingsynchronously moving sliding contacts between which the fineregulationpotentioineter P3 is connected, the contact handle of which isindicated b F1.

At low output potentials, this method of voitage regulation may resultin rather high voltage drops across the stabilizer tubes. To avoidunfavourable working conditions of the stabilizer tubes the abovementioned coarse regulators K1 may be combined wtih a synchronouslymoving regulator in the primary of the high voltage transformer,allowing a simultaneous regulation in steps of the unstabilized highvoltage U11.

11. Special arrangements For many purposes it is necessary to vary thethe potential drops of the stabilizer tubes within very wide limits, upto 56 av, and consequently the variation range of the control gridpotentials being in the order of 1000 volts. In combination Wiihapparatus for X-ray diagnostics at high X-ray tube currents, the gridvoltage variation may be from negative cut off to positive potential ofsome hundred volts, resulting in considerable grid currents. Ordinaryoutput stages of control amplifiers will not fulfill these extremerequirements, the above described amplifiers, however, may be modifiedinto a suitable form with rather small alterations.

According to the present invention the fixed load resistanceinthe'anode' circuit of the'output tube may be substituted by acontinuousvariable resistance, the momentaneous value of which being afunction of the anode current. This is reaiizedby inserting a suitableelectron-tube, acting as a load resistance (here named resistance tubeT11, Figure 8) in the anode circuit of the output tube. In the cathodecircuit of the resistance tube is inserted an ohmic resistance acrosswhich the common anode current ofthe two in series connected tubesproduce a voltage drop being applied to the control grid giving same apotential which is a linear function of the anode current. The cathodeof the resistance tube is connected with the control grid of thestabilizer tub (T2, Figure 8) and its anode is connected to asufiiciently high positive potential relative to the cathode of thestabilizer tube. If the resistance tube is a screen grid tube, a specialfloating screen grid voltage source is needed.

The above arrangement acts as follows: At a sufficiently high anodecurrent in the output tube, the potential drop across the resistancetube attains a high value to the high negative grid potential, the anodevoltage being gradually reduced with the anode current. When the gridpotential of the stabilizer tube be positive, the grid current passesthe resistance tube without disturbing the normal grid contro1 of thetube.

By using suitable resistance tubes with preferably high amplificationfactors, the arrangement is equivalent to a very high resistance in theanode circuit of the amplifier tube, giving the amplifier a practicalvoltage gain very near to the amplification factorof the amplifier tube.In Figure 10 is shown the corresponding arrangement of the amplifier F1in the negative stabilizer.

By means of the above described arrangement the stabilizer canconvenientl be used not only for ordinary symmetrical stabilization butas well to stop the load current effectively by giving the stabilizertubes sufficiently high negative grid potentials. Such an arrangement isshown in Figure 10, in which a magnetically operated switch S1 isinserted in the lead between the anode of the input tube T4 and thecontrol grid of the output tube T5. With switch S1 open the grid of thetube T5 automatically attains, a grid potential giving a very highpotential drop across the resistance tube with the switch closed, theoutput tube attains a grid potential according to the potentiometerratio Eli/R5 giving normal working conditions at constant output voltageE. The magnetic switch is suitably operated by means of thyratrons (R61,R62), which are fired by means of positive pulses applied to the grids.

I claim:

1. A system for stabilizing high tension direct .rcurrent electricalsystems, comprising in combination a high voltage source to bestabilized, two operatively independent grid controlled'stabilizingvalves connected in series in the negative and positive high voltagelead respectively connecting the load with said source, which twostabilizing valves form the stabilizing circuit, a potentiometerconnected across the input of said stabilizing circuit, the variablecontact of which potentiometer is connected tothe input of an electronicamplifier the output of which is connected with the control grid of oneof said stabilizing valves producing a voltage which is a linear andphase-inverted function of the unstabilized input voltage, a furtherpotentiometer connected across the output of said stabilizing circuit,the variable contact of said second p0- tentiometer being connected tothe input of a second'electron'ic amplifier the output of which isconnectedwith the control grid of the other I ofsaid-stabiliz'i-ngvalves for'producing a grldpotential being a direct linear function ofthe stabilized output voltage which is applied to the load.

2. A system as claimed in claim 1 and in which the potentiometerconnected across the input of said stabilizing circuit is permanentlyadjusted (or balanced) according to the equation:

:R2+R3 2 1'l 1 R2 being the part resistance of the potentiometerconnected to the negative terminal of the voltage source, R3 the otherpart resistance connected to the positive terminal, n a factor generallynearly equal to 2, 11 the grain of the amplifier controlling thestabilizing valve in the negative lead, and U1 the amplification factorof said stabilizing tube, the potential drop of said negative partresistance being partly compensated by means of a, constant voltagesource to give a proper input grid bias for said amplifier and byregulating said compensating voltage equal to the mean potential dropacross said stabilizing tube at will Without disturbing the balance ofsaid negative stabilizing system.

3. A system according to claim 1 and in which an ohmic resistance isconnected in the cathode circuit of the negatively connected stabilizingtube, whereby a potential drop occurs across said 10 resistance which isproportional to the output load current being produced, said resistancehaving the value wherein fl is the gain in the amplifier, S1 the mutualconductance of the negatively connected valve, l/m of the potential dropRr-l being applied to the input of the amplifier (F1), so that the gridpotential of the negative stabilizer tube varies approximately linearlywith the anode current in said tube in such a way that within widelimits the potential drop across the tube is nearly independent of theoutput load current.

OLAV N ETTELAND.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,835,121 Rentschyer Dec. 8, 19312,210,393 Braden Aug. 6, 1940 2,394,891 Bowie Feb. 12, 1946 2,434,069Goldberg Jan. 6, 1948

